Angus I am reworking several "N" scale Overton passenger cars and a friend loaned me several of his NTRAK Nn3 books. I have the Nn3 manual and Robert Sloan's Narrow Gauge Data Book. Both have a wealth of information on detailing these cars. They also have drawings and dimensions on the internal floor plans of the individual type cars. Don't know where you can get copies, maybe someone on the Forum can point you in that direction.
Bill